Safety of MT-401-OTS in Patients With Relapsed AML or MDS

Inclusion Criteria: * General 1. Must be ≥ 65 years of age and capable of giving signed informed consent, which includes compliance with the requirements and restrictions listed in the ICF and in the protocol, at the time of signing the ICF 2. Must have a life expectancy ≥ 12 weeks 3. Must have an ECOG performance status of 0-2 4. Must have available MT-401-OTS product with a ≥ 2/8 HLA match Disease Characteristics 5. For participants with AML: 1. Must have a confirmed diagnosis of AML or MDS/AML per 2022 WHO Classification of Haematolymphoid Tumours: Myeloid and Histiocytic/Dendritic Neoplasms or 2022 International Consensus Criteria 2. Must have intermediate or high-risk disease based on ELN 2022 criteria. 3. If no targetable mutation is present, must have received 1 prior standard regimen with at least 4 cycles of standard therapy containing an HMA or a standard cytarabine-containing induction therapy 4. If targetable mutation is present, must have received a regimen that includes commercially available targeted therapy unless unable to tolerate or the participant declines (must be documented in the informed consent). If targeted therapy was not administered as part of first-line of therapy, a second regimen is allowed. 5. Must have either: ≤ 10% bone marrow blasts and ≤ 5% peripheral blasts during screening and not be considered to have hyperproliferating disease at diagnosis or after treatment OR Evidence of MRD based on evaluation at a local laboratory 6. For participants with MDS: 1. Must have confirmed diagnosis of MDS based on 2022 WHO Classification of Haematolymphoid Tumours: Myeloid and Histiocytic/Dendritic Neoplasms or 2022 ICC criteria 2. Must have high-risk or very-high-risk disease based on IPSS-M (ie, not evolved to AML) 3. Must have received standard treatment with at least 4 cycles of an HMA and have evidence of continued disease, including morphologic disease or MRD-positive 4. Must have bone marrow blasts ≤ 10% at screening Health Status 7. Must have adequate coagulation, hepatic, renal, and cardiac function: 1. PT/INR and PTT/aPTT \< 1.3 × ULN 2. AST and ALT \< 3 × ULN; for participants with leukemic infiltration of the liver (documented by biopsy or imaging), AST and ALT \< 5 × ULN is permitted. 3. Total bilirubin ≤ 1.5 × ULN unless bilirubin rise is due to Gilbert's syndrome or of nonhepatic origin (2 × ULN is permitted) 4. eGFR ≥ 40 mL/min by the MDRD formula 5. LVEF ≥ 45% (prior to apheresis and lymphodepletion) Sex 8. Women of childbearing potential are eligible to participate if they agree to the following during the intervention period and for at least 1 year after the last infusion of MT-401-OTS: 1. Must use a contraceptive method that is highly effective (ie, with a failure rate of \< 1% per year; see Section 10.3), preferably with low user dependency PLUS 2. Must agree not to donate eggs (ie, ova and oocytes) for the purpose of reproduction 9. Male participants are eligible to participate if they agree to the following during the intervention period and for at least 6 months after the last infusion of MT-401-OTS: 1. Must refrain from donating sperm PLUS either: 2. Must be abstinent from intercourse as their preferred and usual lifestyle (abstinent on a long-term and persistent basis) and agree to remain abstinent OR 3. Must agree to use a male condom AND should also be advised of the benefit for a nonpregnant female partner to use a highly effective method of contraception (see Section 10.3) as a condom may break or leak Exclusion Criteria: * Disease-Related 1. Have leukemic involvement in the CNS 2. Have other extramedullary disease involvement (except hepatosplenic involvement) 3. Have APL Medical Conditions 4. Have primary immunodeficiency 5. Have severe or uncontrolled autoimmune disorder 6. Have a history or presence of clinically relevant CNS pathology, such as epilepsy, seizure, aphasia, stroke, severe brain injuries, dementia, Parkinson's disease, cerebellar disease, organic brain syndrome or psychosis 7. Have active malignancies (ie, those that are progressing or have required treatment change in the last 24 months) other than the disease being treated under study. Exceptions to this inclusion include the following: 1. Nonmelanoma skin cancer treated within the last 24 months that is considered completely cured 2. Adequately treated breast lobular carcinoma in situ and breast ductal carcinoma in situ 3. Adequately treated cervical carcinoma in situ without evidence of disease 4. History of localized breast cancer and receiving antihormonal agents, or history of localized prostate cancer (N0M0) and receiving androgen-deprivation therapy 5. A malignancy that is considered cured with minimal risk of recurrence 8. Have any active systemic infection requiring therapy (viral, bacterial, or fungal), including HIV 9. Have active hepatitis B or C infection or other clinically active liver diseases, as defined below: 1. Seropositivity for hepatitis B as defined by a positive test for HbsAg Participants with resolved infection (ie, participants who are HbsAg-negative with antibodies to total anti-HBc with or without the presence of anti-HBs) must be screened using RT-PCR measurement of HBV DNA levels. Those who are RT PCR-positive will be excluded. Participants with serologic findings suggestive of HBV vaccination (anti HBs positivity as the only serologic marker) AND a known history of prior HBV vaccination, do not need to be tested for HBV DNA by RT PCR. 2. Active hepatitis C infection as defined by being positive for a nucleic acid test for HCV RNA 10. Have Class III or IV congestive heart failure per New York Association 11. Have unstable angina 12. Have a history or evidence of current, uncontrolled, clinically significant, unstable arrhythmias 13. Have an oxygen saturation on room air of ≤ 92% 14. Have clinically significant reversible nonhematologic toxicities from prior cancer therapy that have not recovered to Grade 1 or baseline Note: Participants with clinically nonsignificant toxicities, such as asymptomatic laboratory values, will be allowed on study. Prior/Concomitant Therapies 15. Received prior treatments for underlying malignancy, except as specified in the Inclusion Criteria. Participants with AML secondary to MDS may have received prior treatment for MDS. 16. Have had prior HSCT 17. Are receiving concurrent therapies other than HMA, as delineated in the study design 18. Have received hematopoietic growth factors within 2 days of lymphodepleting conditioning regimen 19. Have a history of severe allergic reactions/intolerance to any of the study intervention components, including the conditioning regimen, HMA, or DSMO, or to tocilizumab 20. Have had major surgery within 14 days (central line placement allowed) 21. Have received systemic steroids (exception: physiological doses of steroids allowed) or other immunosuppressive therapies within 14 days prior to lymphodepleting conditioning regimen Other 22. Are unable to be matched with MT-401-OTS product inventory 23. Are pregnant or breastfeeding 24. Have any other issue that, in the opinion of the treating physician, would make the participant ineligible for the study or unable to comply with its requirements